The present invention generally relates to a four-stroke cycle internal combustion engine using a mixed fuel with a lubricant added thereto, and more particularly to a four-stroke cycle internal combustion engine suitable for use in a power source of compact power working machines such as a portable trimmer, a lawn mower, a chain saw, or the like.
A portable working machine represented by a trimmer and a chain saw is required to allow an operator to work without any restriction on his working position or posture. Thus, an internal combustion engine or a power source which is mounted on such a working machine is required to maintain stable operation even if it is used, for example, in a laterally tilted position.
In order to comply with such a requirement stated heretofore, there has been generally utilized a compact air-cooled two-stroke cycle gasoline engine (hereinafter referred to as xe2x80x9ctwo-stroke cycle enginexe2x80x9d) which uses mixed fuel of fuel and lubricating oil mixed with each other at a certain ratio. However, since the two-stroke cycle engine generally produces exhaust gas containing much unburnt gas due to a gas-flow type scavenging system employed therein, it has a disadvantage in that it is difficult to take effective measures to control exhaust gas or emission.
From the standpoint of emission control, a four-stroke cycle internal combustion engine (Otto engine) has the advantage of producing exhaust gas containing a small amount of unburnt gas, whereby it is desirable for the four-stroke cycle internal combustion engine to be employed in the portable working machine in place of the two-stroke cycle engine. In the four-stroke cycle engine, typically, an oil reservoir is disposed in a bottom section of a crankcase in which a crankshaft is accommodated. The oil in the oil reservoir is drawn by a pump and/or is splashed by a rotating member to lubricate an inner surface of a cylinder and a valve system. However, this likely results in a complicated structure and an increased weight due to an additional mechanism for oil delivery and recovery, which is undesirable for the portable working machine.
An existence of the oil reservoir forces the operator to keep the engine in a specified position (generally to keep the cylinder in an upright position) in use, which makes it difficult to apply a typical four-stroke cycle engine to the portable working machine without a modification. Further, there is a possibility of engine seizure due to an unstable or insufficient lubricating oil supply caused by a change in a position or posture thereof. This may interfere with an operational convenience as a portable working machine. Conventionally, various proposals have been submitted in order to improve such problem, one of which is to use mixed fuel of fuel and lubricating oil in the four-stroke cycle engine as in the two-stroke cycle engine.
In the four-stroke cycle internal combustion engine using the mixed fuel, as disclosed in, for example, Japanese Patent Laid-Open Disclosure Nos. Hei 5-222944 and Hei 7-150920, U.S. Pat. Nos. 5,347,967 and 5,579,735, an intake system device including a carburetor is directly connected to a crankcase defining a crank chamber. A mist-like mixed fuel which includes a lubricating oil and which is introduced through the intake system device into the crankcase lubricates an inner surface of a cylinder and rotating members in the crank chamber. It is then introduced into a cylinder head through a communicating passage which connects the crankcase and a cylinder head and lubricates a valve mechanism of intake and exhaust valves in the cylinder head. It is further introduced into a combustion chamber through an intake port.
Employment of the four-stroke cycle internal combustion engine using the mixed fuel allows elimination of the oil reservoir in the bottom section of the crank chamber, an independent oil supply device (a pump or a member to splash the lubrication oil) and also an oil recovery device. Therefore, it provides several advantages that, for example, the stable operation of the engine can be obtained even if it is used in various positions or postures. It further provides advantageous features of lightweight and simple structure for the engine to be used for the portable working machine.
Another type of four-stroke cycle internal combustion engine using the mixed fuel is also disclosed in Japanese Patent Laid-Open Disclosure No. Hei 11-223116. This publication discloses an invention related to an OHV type engine, wherein a mixed fuel including a lubricating oil is introduced into a crankcase through a branch intake passage branched from an intake passage to lubricate inner surfaces of a cylinder and rotating members in a crank chamber, and is then led from the crankcase to a cylinder head through a space for a push rod of an OHV mechanism to lubricate a valve mechanism in the cylinder head. It is then returned to the intake passage.
In the four-stroke cycle internal combustion engine whose inside is lubricated by using a mixed fuel including a lubricating oil, the negative effect upon emission gas caused by the introduced lubricating oil is not negligible since much lubricating oil is introduced into the combustion chamber with a fuel. In addition, there is a possibility of deteriorating engine performances due to the deposit on valve seats of intake and exhaust valves and/or a spark plug resulting from the lubricating oil which is exposed to a high temperature in the combustion chamber.
The present invention is directed to a four-stroke cycle internal combustion engine using a mixed fuel including lubricating oil, and intends to provide a four-stroke cycle internal combustion engine capable of reducing the amount of lubricating oil which is to be introduced into a combustion chamber with a fuel.
According to the present invention, the above object can be achieved by providing a four-stroke cycle internal combustion engine whose inside is lubricated using a mixed fuel with a lubricant added thereto, said four-stroke cycle internal combustion engine comprising: a combustion chamber; and a path extending from a point at which said mixed fuel is introduced into said engine to a point before charging said mixed fuel into said combustion chamber, whereby a fuel component included in said mixed fuel is vaporized by heating it in said path utilizing heat released from said engine.
Thus, according to the present invention, since the mixed fuel including a lubricating oil is heated by the heat released from the engine, the fuel component in the mixed fuel is vaporized and separated from the lubrication oil before the mixed fuel is introduced into the combustion chamber. It allows the lubrication oil to stay within the engine without discharging is therefrom to outside. In other word, by vaporizing the fuel component in the mixed fuel in the passage introducing it into the combustion chamber, the amount of lubrication oil which is to be included in the mixed fuel introduced into the combustion chamber may be considerably reduced.
In a specific embodiment of the present invention, a first communicating passage which allows fluid communication between a crank chamber accommodating a crankshaft and a valve chamber provided in a cylinder head may be formed in a cylinder block to vaporize the fuel component included in the mixed fuel by heating the mixed fuel passing through the first communicating passage. As is known, the crank chamber requires larger amounts of lubricating oil as compared to the valve chamber in the cylinder head. From this point, it is reasonable to vaporize the fuel component by heating the mixed fuel with the heat released from the cylinder block during the movement of the mixed fuel from the crank chamber to the valve chamber.
Preferably, a second communicating passage which allows fluid communication between the valve camber and the crank chamber is provided to return the lubrication oil in the valve chamber to the crank chamber, so that the amount of lubrication oil in the crank chamber may be maintained in rich condition.
The present invention may be applied to a certain type of engine in which an intake system device is directly connected to a crankcase as disclosed in Japanese Patent Laid-Open Disclosure No. Hei 5-222944 described above. It may be also applied to another type of engine in which a mixed fuel including a lubrication oil is introduced to a crankcase through a branch intake passage branched from an intake passage as disclosed in Japanese Patent Laid-Open Disclosure No. Hei 11-223116 described above.
As for engine types, the present invention may be applied not only to OHC type engines but also to OHV type engines. In the case of the OHV type engines, a hole through which a push rod forming a part of a valve mechanism is passed may be used as the communicating passage for introducing the mixed fuel from the crank chamber to the valve chamber of the cylinder head, wherein the hole may be formed in a cylinder block to vaporize the fuel component by heating the mixed fuel during the movement of the fixed fuel from the crank chamber to the valve chamber.
Other objects and operational advantages of the present invention shall be apparent from the following description of the preferred embodiment.